This invention relates to a radiation-measuring apparatus capable of analyzing the energy of radiations such as .alpha.-, .beta.-, .gamma.- and X-rays, determining radiation intensity and detecting the position of a radiation source.
With the prior art radiation-measuring apparatus designed to detect radiation by means of a scintillator, .gamma.-rays, for example, interacting with the scintillator are converted into scintillations; and the produced scintillations are converted into current by a photomultiplier tube (hereinafter referred to as "PMT"), and then into current pulses by a current amplifier. The current pulses are integrated by an integrator. The waveform of the integrated current pulses is clipped by a clipped pulse generating circuit, producing a current pulse of short width. The clipped pulse is supplied to a processing circuit comprising, for example, a pulse-height analyzer, thereby determining the energy and intensity of detected radiations. In this case, the pulse-height of an integrated pulse which has been clipped to a short width by the clipped pulse generating circuit is not proportional to the total amount of a scintillation sent forth from the scintillator, but to that portion of the scintillation which has been given off during the clipping period t.sub.c. With conventional radiation-measuring apparatus, an attempt to reduce the clipping period t.sub.c in order to shorten a period of analyzing radiation energies would lead to increased statistical fluctuation in the pulse-height. This statistical fluctuation in the pulse-height becomes one of the factors of determining the energy resolving power of a detection system. Therefore, the shortening of the clipping period conversely decreases said energy resolving power, though improving the time resolution of the detector system.
There will now be described the above-mentioned relationship by a mathematical formula. With T taken to denote the decay time constant of the scintillation, and N the total number of photoelectrons collected at the first dynode of the PMT, then the statistical average number .DELTA.N of photoelectrons gathered during a period extending from the point of time t immediately after the incidence of the radiation into the scintillation detector to the point of time (t+.DELTA.t) may be expressed by the following formula (1): EQU .DELTA.N=(N/T)e.sup.-t/T .multidot..DELTA.t (1)
Therefore, the statistical average number N.sub.m of photoelectrons collected during the clipping period t.sub.c may be expressed by the following formula (2): EQU N.sub.m =N(1-e.sup.-t.sbsp.c.sup./T) (2)
Since the actually observed number of photoelectrons statistically fluctuates in accordance with Poisson's distribution, the relative standard deviation R in a number of photoelectrons may be expressed by the following formula (3): ##EQU1## The relative standard deviation R expressed by the equation (3) acts as a guide in indicating the energy resolving power. This relative standard deviation R may be expressed as R=1/.sqroot.N when the clipping period t.sub.c is long, and becomes equal to that which arises when the total light quanta are collected. Where, however, the clipping period t.sub.c is shortened, then the relative standard deviation R increases accordingly, resulting in a larger statistical error in a measured value and consequently a decline in the energy resolving power of a radiation-measuring apparatus.
With a scintillation detector in actual use, for example, the NaI (T1) scintillator, the decay time constant T of the scintillation indicates 0.25 microseconds. With (t.sub.c /T) chosen to have a value of 3 to 4, then the clipping period t.sub.c is about 0.75 to 1.0 microsecond, and the clipped pulse has a width of about 1 to 1.5 microseconds. This pulse width is too broad to shorten the resolving time and unadapted for use at a high count rate.
As mentioned above, the conventional radiation-measuring apparatus has the drawbacks that where the clipping period t.sub.c is made long to elevate the energy resolving power, then the time resolution is subject to a certain limitation and the apparatus cannot operate effectively at high count rates. Conversely, where the clipping period t.sub.c is shortened to decrease the resolving time, then the energy resolving power drops, thus bringing about contradictory results. Therefore, a present need exists for a radiation-measuring apparatus having both improved energy resolving power and improved resolving time.